1. Field of the Invention
The present invention relates to a control device for an a. c. generator for an automobile. More particularly, it relates to such a device for preventing breakage of a transistor for actuating an alarm device when a fault such as short-circuitting in the alarm device occurs.
2. Discussion of Background
FIG. 2 is a diagram showing a conventional control device for an a. c. generator. In FIG. 2, an a. c. generator 1 driven by an engine (not shown) mounted on an automobile comprises an armature winding 101 and a field winding 102. The a. c. generator 1 produces an alternating current output, which undergoes full wave rectification by a rectifier 2 having three output terminals 201, 202, 203.
The first output terminal 201 is the main output terminal for outputting a main output; the second output terminal 202 is an autiliary output terminal which excites the field coil 102 of the a. c. generator 1 and applies a rectified output voltage of the a. c. generator 1 to a voltage regulator 3 and an alarm device 4, and the third output terminal 203 is a ground terminal.
The voltage regulator 3 controls the output voltage of the a. c. generator 1 to be a predetermined value. The voltage regulator 3 comprises two voltage-dividing resisters 301, 302 which divides the output voltage at the autiliary output terminal 202. The connecting point between the voltage-dividing resisters 301, 302 is connected to the base of the transistor 304 through a Zener diode 303, which detectes the output voltage of the a. c. generator 1. When the Zener diode 303 is turned on, the transistor 304 is also turned on. When the Zener diode is turned off, the transistor 304 is also turned off.
The transistor 304 has its emitter grounded and its collector connected to the base of an output transistor 305. The base of the output transistor 305 is connected to the second output terminal 202 of the rectifier 2 through a resister 306. The emitter of the transistor 305 is grounded and the collector is connected to the second output terminal 202 through a diode 307.
The transistor 304 controls the turning-on and off of the output transistor 305, which in turn controls a field current in the field coil 102. The diode 307 is connected to the collector of the transistor 305 in parallel to the field coil 102 to absorb a surge current produced in the field coil 102.
The alarm device 4 which detectes the output voltage of the a. c. generator 1 and generates an alarm, comprises an alarm lamp 9 and an alarm lamp actuating transistor 401 and a battery 5, wherein the collector 5 of the actuating transistor 401 is connected to the positive terminal of the battery 5 via the alarm lamp 9 and a key which 6, and the negative terminal of the battery is grounded. The emitter of the actuating transistor 401 is grounded and the base of the transistor 401 is connected to the collector of a controling transistor 402. The controlling transistor 402 is used to control the alarm lamp actuating transistor 401 and it has the emitter grounded and the collector connected to the base of the actuating transistor 401. The base of the actuating transistor 401 is connected to a connecting point for connecting the key switch 6 to a reverse current blocking diode 7 through a resister 404.
The base of the controlling transistor 402 is connected to one end of the field coil 102 through a Zener diode 403 and a resister 405.
The positive terminal of the battery 5 is connected to one end of the field coil 102 through the key switch 6, the reverse current blocking diode 7 and an initial-exciting resister 8 which controls excitation of the field coil 102 at initial stage.
The operation of the conventional control device will be described. When the key switch 6 is closed to start the engine, an initial-exciting current is fed from the battery 5 throught the key switch 6, the reverse current blocking diode 7 and the initial-exciting resister 8 to the field coil 102 whereby the a. c. generator 1 generates power. At this moment, potential at the autiliary output terminal 202 in the rectifier 2 takes a value obtained by dividing the voltage of the battery 5 by the initial-exciting resister 8 and the resister of the field coil 102, the value being relatively low.
A base current is supplied from the battery 5 through the key switch 6 and the resister 404 to the base of the alarm lamp actuating transistor 401 to turn on the same. When the transistor 401 becomes conductive, the alarm lamp 9 is lit. This displays that the a. c. generator generates no power.
When the engine is started to actuate the a. c. generator 1 for generation of power, the output voltage of the generator 1 is applied to the Zener diode 403 through the resister 405. When the generated voltage is increased to a predetermined value, the Zener diode 403 is turned on, whereby the controlling transistor 402 becomes a conductive state. Then, the alarm lamp actuating transistor 401 is turned off and the alarm lamp is turned off. Thus, the alarm device 4 displays that the a. c. generator 1 starts power generation.
In the voltage regulator 3, the voltage-dividing resisters 301, 302 and the Zener diode 303 detect the output of the a. c. generator 1, and when the output voltage of the generator 1 exceeds a voltage level determined by the resisters 301, 302 and the Zener diode 303, the Zener diode 303 is turned on to effect the transistor 304.
On the contrary, when the output voltage of the a. c. generator 1 is at more than the predetermined voltage level, the Zener diode 303 is turned off, whereby the transistor 304 is also turned off.
Thus, the output transistor 305 is turned on or off depending on the condition of the transistor 304 to thereby control a field current flowing in the field coil 102, with the result that the output voltage of the a. c. generator 1 is adjusted to be a predetermined range of voltage. In the convetional control device as described above, the alarm lamp 9 is lit when the key switch 6 is closed to start the engine, or when the output voltage of the a. c. generator 1 is decreased by any cause during the operation of the engine. Under the condition, when there takes place a short circuit in the alarm lamp 9 by any cause, a large short-circuit current flows in the alarm lamp actuating transistor 401. The large short-circuit current sometimes breaks the transistor. When the voltage of the battery 5 is erroneously supplied directly to the collector of the alarm lamp actuating transistor 401 during, for instance, maintenance works for the control device, a trouble similar to that as describe above takes place to thereby render the alarm device 4 to be inoperable. Thus, in the convention of control device for the a. c. generator for automobiles, there was such a drawback that the alarm device 4 becomes faulty by breakage of the alarm lamp actuating transistor 401 when a short-circuit takes place in the alarm lamp during the operation of the alarm lamp or there was an error in maintenance works.